Method for modifying steam or hybrid propulsion equipment for LNG carriers

Abstract

A method for retrofitting an installation of a type used for a hybrid propulsion system for LNG carriers, said method being based on the steps of retrofitting an LNG carrier boiler of the installation, retrofitting an existing exhaust gas outlet and integrating a new exhaust gas outlet, forming an opening in the combustion chamber of said LNG carrier boiler, introducing exhaust gas from a GTPG inside the formed opening and adding a second superheater inside the exhaust gas outlet between said GTPG and said LNG carrier boiler, integrating an auxiliary steam and water system and integrating an automatic changeover extraction system and connecting both systems to said hybrid propulsion installation.

Description

Detailed Description of the Invention

[0001] [Object of the Invention] The object of the present invention relates to a method for modifying a steam turbine propulsion system, a hybrid steam-diesel propulsion system, or a hybrid steam-gas turbine propulsion system for an LNG (Liquefied Natural Gas) carrier.

[0002] In particular, the object of the present invention relates to steam turbine propulsion installations that are required by recent IMO regulations to operate at low loads, and hybrid propulsion installations that use marine boilers and steam systems, including ships that have been retrofitted with hybrid propulsion to reduce emissions while maintaining or reaching an operationally optimal speed level. The object takes into account the risk of corrosion from sulfur dioxide at low exhaust gas temperatures, and modifies the steam cycle when the ship's steam plant operates at low loads to restore steam cycle efficiency and save a considerable amount of steam, and therefore fuel, in an economical and efficient manner.

[0003] BACKGROUND OF THE INVENTION The types of propulsion used on LNG (liquefied natural gas) carriers have undergone several major changes over the past 15 years.

[0004] Since the first LNG carrier was built about 55 years ago, until about 15 years ago, virtually all LNG carriers used steam turbine propulsion systems.

[0005] This is a very reliable propulsion system, but its efficiency is significantly lower than diesel-engine based propulsion systems. In fact, only LNG carriers use steam turbine propulsion today.

[0006] The main reason for this is the sufficient capacity of ship's boilers to use boil-off gas (BOG) generated by the evaporation of LNG cargoes as fuel. Liquid fuels such as high sulfur heavy fuel oil (HSHFO) were also used in combination in any proportion depending on the requirements and convenience of the voyage.

[0007] Main steam boilers of the kind normally used in the propulsion of steam turbine LNG carriers, including, more frequently, boilers incorporating the following major components: 1 steam drum 1 water drum 2 superheated steam drums · A number of superheated steam pipes, usually vertically mounted and U-shaped. · Several furnace screen water tubes located in one or more rows between the furnace and the superheater. A number of water tubes arranged in several rows, connecting the water drum with the steam drum, located after the superheater tubes and collectively named the main generating bank. One high-pressure feedwater economizer located at the top inside the exhaust gas outlet. -Multiple water wall bottom headers located at the bottom of the furnace. ·One furnace where all walls are restricted by water wall tubes except for one wall where a screen water tube is installed. A combustion air chamber and its air inlet, located above the furnace and protected at the bottom by a water wall tube. - Multiple burners, most often three, located at the top of the furnace and entering through the combustion air chamber. One economizer top drum water inlet. -One economizer bottom drum water supply outlet. One exhaust gas duct outlet located above the top of the economizer. - Numerous water wall pipes running from the water wall bottom header to the water wall top header. Some of those water wall pipes run from the front water wall bottom header to the front water wall top header. Multiple water wall top headers.

[0008] Over the past few years, the use of HSHFO has been gradually restricted and eventually banned by the International Marine Organization (IMO). HSHFO has been replaced by low sulfur heavy fuel oil (LSHFO) and / or marine diesel oil (MDO), and even ultra-low sulfur marine diesel oil (ULSMDO). At the same time, and for the same reasons, the proportion of BOG used has also been continuously increasing.

[0009] Currently, about one-third of the existing LNG carrier fleet is still steam turbine powered. There are currently about 224 steam turbine powered LNG carriers in operation, which is a very large number of vessels.

[0010] Since 2007, the propulsion trend has begun to change dramatically, with dual-fuel diesel-electric propulsion, based on 3-5 dual-fuel diesel generators and 2 electric propulsion motors, becoming the dominant propulsion type. Steam turbines have been rapidly declining, although some ships are still being built, mainly in Japan.

[0011] Again, the dominant propulsion type changed just 10 / 12 years later, around 2017 / 2018, with two-stroke, slow-speed, direct-drive, dual-fuel diesel engines now becoming the preferred choice.

[0012] Such rapid and accelerating change is extremely rare in marine propulsion systems, and in fact is driven in part by the growing importance of greenhouse gas (GHG) emissions and the subsequent demand for increased efficiency.

[0013] As a result of this rapid change, steam turbine LNG carriers are increasingly being viewed as inadequate vessels. However, many of these vessels are still in the middle of their economic lifespan. Shipowners are being forced to explore the types of modifications that can improve the capabilities of their vessels in order to survive in the current market. However, so far, no propulsion modifications have been undertaken.

[0014] To further complicate matters, the International Maritime Organization (IMO) has taken regulatory measures to impose strict cuts in emissions, amounting to a roughly 30% reduction in GHG (Green House Gas emissions).

[0015] A preliminary decision was made by the Marine Environment Protection Committee (MEPC) in November last year, and it will be approved and developed at the next MEPC meeting in June 2021.

[0016] Compliance will be required for the date of the corresponding survey after 1 January 2023. In addition, some type of classification of ships will be established depending on the efficiency of their propulsion equipment.

[0017] Thus, ships are classified as A, B, C, D, and E, with Class A being the most efficient and Class E being the least efficient. The IMO is likely to require efficiency improvements for D and E class ships under a fuel efficiency performance rating system (Carbon Intensity Index, CII). It is likely that most, if not all, steam turbine LNG carriers will be classified as D or E.

[0018] Currently, after the IMO's new rules, the state of the steam turbine LNG carrier fleet is such that around 224 of the existing steam turbine ships are nearing the middle of their lifespans and are due to reduce emissions by 30% by 2023.

[0019] It is important to note that the final details of the new rules have yet to be finalized and regulations may have different requirements, but in any case, there will be new and stricter regulations on reducing GHG emissions.

[0020] Conventional steam turbine marine power plants are designed so that as power output is reduced, power plant efficiency also decreases at an increasingly rapid rate. For example, when shaft horsepower is reduced by 50%, power plant efficiency decreases by approximately 25%.

[0021] This dramatic decrease in efficiency is not inherent in the steam turbine itself, but rather is a result of the way the complete cycle is designed. Steam cycles have been designed specifically for high-power operation, and low-speed steam operation has essentially been ignored. As a result of new regulations and the way existing steam cycles are designed, existing steam turbine-powered LNG carriers are required to operate at less than approximately 50% of their design shaft horsepower unless their propulsion plant is modified.

[0022] As mentioned above, the use of HSHFO has been prohibited for several years. Currently, the fuel is mainly boil-off gas and, to a much lesser extent, LSHFO or ULSMDO.

[0023] Both changes in fuel quality and GHG emissions limits pave the way for modifying existing facilities in several ways.

[0024] As things stand, owners of steam turbine LNG carriers are faced with decisions on how to bring their ships into compliance with the IMO's new rules.

[0025] However, several problems arise when attempting to convert steam turbine propulsion equipment for LNG carriers, one of which is the efficiency lost by operating at lower loads to reduce emissions, because the conventional propulsion equipment was not designed to operate at those loads.

[0026] Another problem that arises when emissions have to be reduced is that when LNG carrier owners reduce the shaft horsepower (SHP) of their vessels, the speed of the vessels is also significantly reduced, making them incompatible with the speed requirements set by the TCH parties (time charterers).

[0027] [Details of the invention] The present invention relates to a method for modifying the steam or hybrid propulsion system of a liquefied natural gas carrier to reduce the exhaust gas emissions required by IMO while maintaining the efficiency of the vessel or at least reducing the efficiency to an acceptable level and reducing the fuel consumption of the steam or hybrid propulsion system.

[0028] The method that is the subject of the present invention seeks to solve the above-mentioned problems by retrofitting the steam or hybrid propulsion installation of an LNG carrier.

[0029] The method includes a series of steps that can be applied to new or existing installations. By applying the method of the present invention, a carrier vessel maintains its speed, limits its speed loss, or reaches an optimal speed according to the TCH contractor's requirements, while reducing emissions to IMO required standards.

[0030] In addition, the method of the present invention has two distinct goals: One of these is to keep ships in compliance with the new IMO requirements on EEXI and CII for a set period of time (due to the progressive nature of the IMO requirements).

[0031] This goal means reaching a certain value of EEXI as defined and required by IMO. To reach the required value, one or more proposed efficiency improvement methods may be applied until the ship's achieved value of EEXI meets the IMO requirements. The same applies to CII, with the additional requirement of incremental improvement over many years.

[0032] A second goal is to make ships more commercially attractive by reducing their energy consumption.

[0033] A first aspect of the invention involves a method of incorporating a low pressure economizer into a marine boiler of the type used with existing steam or hybrid propulsion drives on LNG carriers so that the propulsion system can be operated at low loads and obtain levels of efficiency approaching the level of the propulsion system when operated at high loads.

[0034] Exhaust gases from conventional boilers in marine propulsion plants of this type leave the boiler through existing exhaust gas ducts.

[0035] A high-pressure economizer resides in the exhaust gas duct and uses heat from the exhaust gas to raise the temperature of the boiler feedwater, thereby reducing fuel used and increasing the efficiency of the equipment.

[0036] A first embodiment of the method is characterized in that the exhaust gas duct is modified so that a low-pressure economizer is placed after the main economizer, and the water supply to this low-pressure economizer is from the main condenser extraction water circuit, where the water temperature is lower, making the heat exchange in the low-pressure economizer more efficient.

[0037] The introduction of a low-pressure economizer into a facility can be done as follows: by installing it in series with the existing duct or by bypassing it. In this case, there are two options: The economizer is installed either as a fully prefabricated module outside the existing engine housing, or it is installed inside the engine casing on top.

[0038] Here too, the heat of the exhaust gases leaving the main economizer at a temperature of 150-175°C is used to raise the temperature of the boiler feedwater circulating through the low-pressure economizer by heat exchange with the exhaust gases so that the exhaust gases leave the boiler at a final temperature of less than 100°C.

[0039] The design and configuration of the low-pressure economizer allows for this heat recovery and lowers the exhaust gas temperature. Depending on the design, it may also be possible to condense some of the water vapor present in the exhaust gas. In this case, the amount of heat recovered is significantly increased. The low-pressure economizer is designed so that the condensed water is recovered and sent outside the exhaust gas flow. This additional heat recovery is possible due to the low temperature of the main condenser extraction water circulating inside the low-pressure economizer. In fact, the materials and design of the low-pressure economizer must be sufficient to withstand potential corrosion at low temperatures if sulfur is present in the exhaust gas.

[0040] Finally, low-pressure economizers can be installed on both boilers or just one. Almost all steam turbine LNG carriers have two main boilers.

[0041] However, as explained earlier, the installation of a second economizer is linked to operating a steam power plant with low steam velocity, so retrofitting a second economizer to only one of the two existing boilers can be a very good option to reduce costs and installation time. In this case, one possibility is to operate the steam plant with only one boiler.

[0042] If a new generator (gas turbine or DF diesel generator) is also retrofitted as part of compliance with IMO emissions reductions, the steam plant will operate at low steam speed most of the time and only one boiler firing will be required. Also, due to IMO emissions limits, it is possible to operate only one boiler at around 50% capacity.

[0043] A second aspect of the invention involves a method based on retrofitting one or both of an existing boiler with a heat recovery steam generator (HRSG).

[0044] A second aspect of the method involves modifying one surface of a water tube wall of an existing boiler furnace by cutting a portion of the water tube to form an opening in the selected water tube wall to allow exhaust gas from a gas turbine generator or diesel engine generator to enter the boiler furnace, so that the existing marine steam boiler can operate as a heat recovery steam generator (HRSG) while maintaining the ability to operate as a dual fired boiler when needed, and can operate in both modes simultaneously.

[0045] The water pipes cut to form flue gas openings for the inflow of flue gases are replaced by new, specially bent pipes of a different design, welded at both ends to the previously cut sections of the water pipes, maintaining the continuity of water circulation in all the pipes of the boiler furnace water wall.

[0046] The shape of the new water pipe bend is designed in three dimensions as follows: Ensure water circulation in all pipes; Maintaining the airtightness of water pipe walls; The new section of water pipe is then overlapped with the remaining water pipe to form an opening for the inflow of exhaust gases and / or a duct for the generator exhaust gases that directs the flow of such gases in the required direction.

[0047] In this way, one or both of the existing steam boilers may be operated as fired boilers and / or as HRSGs, and / or both ways simultaneously.

[0048] Another option for modifying one side of an existing boiler is to introduce gas turbine or diesel engine exhaust gases into the combustion air duct after the forced draft fan and before the combustion air inlet to the furnace. This is done so that the exhaust gases flow through the existing openings in the roof water tube panel at the top near the burner openings, so that they flow through the existing boiler in the same way as the combustion gases before the U-flow conversion. The ducts can be installed with any conventional fittings and configurations.

[0049] An advantage of the second aspect of the present invention is that an existing boiler can be used as an HRSG while retaining its ability to be used as an MPMB or simultaneously fired and used as an HRSG.

[0050] In this way, the cost of a new HRSG and the cost and time required for retrofitting are saved.The MPMB can perform satisfactorily as an HRSG if its heat exchange capacity is suitable for the amount and conditions of the gas turbine (GT) exhaust gas.

[0051] This heat exchange capacity must be verified by appropriate calculations. In fact, the amount of steam generated corresponds to the amount and temperature of the exhaust gases.

[0052] On the other hand, GT fuels should be either sulfur-free boil-off gas or ULSGO under all normal conditions, so there is no risk of corrosion from low-temperature exhaust gases.

[0053] The second aspect of the present invention is also a method supplemented by a third aspect of operation comprising retrofitting an additional steam superheater located in a newly retrofitted flue gas duct before the gases enter the furnace of an existing boiler through a formed opening.

[0054] The new steam superheater is connected with appropriate valves to the existing superheated steam main, with its inlet connected to the outlet of the existing superheater and its outlet connected to the boiler superheated steam main before being connected to the other boiler superheated steam main.

[0055] This connection then supplies superheated steam to the main turbine and other major users of superheated steam.

[0056] In this way, the temperature of the superheated steam is higher than can be achieved with the existing superheaters of the existing main boilers, and reaches temperatures equal to or approaching the temperature of the superheated steam in existing dual gas and liquid fuel steam boilers.

[0057] The new steam superheater will be constructed from similar materials (high temperature steel alloys) as those used in the existing superheaters and will consist primarily of superheating steam pipes, superheated steam recovery vessels, and the necessary valves and controls.

[0058] If the boiler is operated as a fired boiler, the new superheater module can be isolated if necessary, in which case a small amount of steam circulation may be maintained using appropriate equipment to maintain acceptable temperatures within the new superheater tubes when exposed to residual levels of radiation and convection without the main steam circulation.

[0059] A fourth aspect of the present invention is based on the integration of existing and new auxiliary steam and water systems that introduce steam and / or heated water produced in the low-pressure exhaust boiler of a conventional installation into selected piping points as required when operating as a HRSG, which can be connected to an existing deaerator on the LNG carrier, or the low-pressure exhaust boiler can operate independently.

[0060] The steam circulating inside the low-pressure exhaust boiler is heated by the exhaust gases of a new dual-fuel generator, usually a diesel generator (DFDG), installed as part of the conversion to form a hybrid propulsion system.

[0061] The relatively low energy steam and heating water produced in the HRSG is used for one or more of the following purposes: to heat the main condenser bleed water before it enters the deaerator; to generate steam for injection into the deaerator; to heat the main boiler combustion air in existing air heaters; to inject steam into the crossover after the high pressure turbine (HPT) and before the low pressure turbine (LPT); and other miscellaneous auxiliary steam uses.

[0062] Generally, all miscellaneous applications requiring low pressure saturated or superheated auxiliary steam HRSG can be performed with steam generated in a low pressure HRSG boiler.

[0063] The savings brought about by such consolidation come from: Reduction of the pre-modification high demand for live steam generated in the main boiler, which is used after one or more pressure reductions for multiple heating demands derived from the closed steam of the main turbine extraction system when operated at low loads to reduce gas emissions. The presence of a large amount of heat at moderate temperatures in the exhaust gases of new generators without the need for additional fuel or gas consumption. The presence of large amounts of hot water, up to 120°C, in newly installed diesel engines in the jacket cooling water system, sufficient to heat the lower temperature condenser bleed water. The integrated system is relatively simple and inexpensive, using much of the existing piping system and heat exchangers, with new steam and / or hot water supplies available from HRSGs and / or diesel generators being retrofitted as part of the new hybrid propulsion scheme.

[0064] The main advantages of using a deaerator as a steam / water separator in a new HRSG are: -Since it is an existing part, its manufacturing and installation costs are zero. The deaerator is already connected to the discharge of the main condenser extraction pump. Such discharge piping will be modified to include circulating water through a new HRSG economizer and discharging to the deaerator. In this way, the main condenser extraction water will be heated to or close to the required temperature. The HRSG forced water circulation pump can easily suck water from the bottom of the deaerator, form a circulation through the evaporator part of the HRSG and discharge to the deaerator where the water and steam mixture separates. The superheater section of the HRSG is fed from the top of the deaerator (or a suitable existing steam line already directly connected to the steam section of the deaerator). The superheated steam HRSG outlet is connected to a different steam user where superheated steam is preferred, for example through a new connection to a crossover between the injected HPT and LPT. The deaerator is already connected to many steam systems. Thus, the connection is made in a very simple way.

[0065] Additionally, any water and steam drums require levels, pressure gauges, and controls. Deaerators already have such systems installed, fully integrated with existing equipment.

[0066] In either case, the integration of steam and hot water produced in the new HRSG can be achieved by using a deaerator or by installing a new drum to separate the steam and water as part of a retrofitted low-pressure HRSG.

[0067] An additional application of this embodiment is the use of the detailed system in a two-pressure steam system as part of a combined cycle steam and gas turbine power generation system.

[0068] The low-pressure system uses a deaerator as a steam drum. The low-pressure steam produced in the low-pressure evaporating section of the HRSG is separated in the deaerator, superheated in the HRSG's low-pressure superheater, and then injected into the low-pressure turbine (LPT) through a connection in the existing crossover between the HPT and LPT.

[0069] Thus, implementing a two steam pressures combined cycle has the advantage of using existing elements such as deaerators, LPTs, and crossovers, and fully integrating existing and new components in a simple and economical manner.

[0070] The invention can be completed by a fifth aspect which includes implementing an automatic switching steam extraction system in such a way that when the ship is operating at low load, steam from the first extraction of the main high pressure turbine is automatically switched to the second extraction steam system in a certain load range.

[0071] In this way, the different steam usage of the second extraction unit is not supplied directly by the desuperheater live steam coming directly from the main boiler, but by the first extraction steam after passing through one or more automatic pressure reducing valves.

[0072] The system is integrated with a number of automated valves and a branch pipe connecting the first extracted steam piping system to the second extracted steam piping system.

[0073] Existing and newly installed automatic valves are sequentially closed and / or opened in a programmed system driven by parameters directly linked to the main turbine load.

[0074] Necessary valve interlocking is included to avoid simultaneous steam ingress from two different automatic valves, and pressure control valves, if necessary.

[0075] Similarly, this embodiment can be applied to switching steam from being supplied by a second extracted steam system from the crossover between the HPT and LPT to being supplied by a third extracted steam system from the LPT.

[0076] At a selected load, where the pressure at a given steam extraction is too low to feed that extraction steam extraction system, but sufficient to feed the next lower pressure steam extraction system, the switchover is activated. At a lower load, where the pressure is again too low to feed the lower pressure steam system, the extraction system is shut down.

[0077] The switching system is typically applied simultaneously with and complements the first aspect of the invention with a low pressure economizer, or in a completely independent manner. The system may be applied to switch between one set of extractions (first to second extractions), or two sets of extractions (first and second to third extractions), as deemed convenient.

[0078] The first aspect of the invention is directly applicable to steam propulsion equipment, improving efficiency at all loads, but particularly at low loads, to compensate for efficiency losses due to operation at low loads.

[0079] The second aspect of the invention can be directly applied to hybrid propulsion systems when retrofitted with GT (gas turbine) or DFDG (dual fuel diesel generator) to improve steam cycle and overall efficiency, contributing to increasing the power and speed at which a ship can operate while still complying with IMO EEXI regulations.

[0080] Although both aspects may be applied independently, the first aspect of the invention may also be complemented by the second aspect in order to reach optimum performance in terms of exhaust emissions.

[0081] Furthermore, the second aspect can be complemented by the third aspect of the invention to also achieve optimum operating requirements in terms of exhaust emissions.

[0082] In addition, a fourth aspect, which can be applied to hybrid propulsion installations when retrofitting with DFDG or GT, can be applied as an alternative to the second and third aspects of the present invention, using the low-pressure exhaust boiler of a conventional installation connected with the existing steam system of the LNG carrier through a deaerator or a new steam / water drum.

[0083] Finally, the fifth aspect of the invention, which is directly applicable to steam turbine propulsion installations, can be applied independently or in conjunction with the other aspects of the invention to support the previous aspects, particularly the first aspect of the invention, to complement improvements in the efficiency of steam power plants at low loads.

[0084] DESCRIPTION OF THE DRAWINGS To complete this specification and to facilitate a better understanding of the features of the present invention, according to a preferred embodiment thereof, a set of drawings is attached as an integral part of said specification, in which, by way of illustrative and non-limiting features, the following are depicted: [Figure 1] This shows a modified exhaust gas outlet from an existing LNG carrier boiler.

[0085] [Figure 2] Modified internal tubes in the combustion chamber of an LNG carrier boiler.

[0086] [Figure 3] A cross-sectional view of the internal tubes of the combustion chamber of an LNG carrier boiler.

[0087] [Figure 4] Shows the superheater integrated in front of the LNG carrier boiler.

[0088] [Figure 5] Shows the integration of auxiliary steam and water systems within an LNG carrier propulsion system.

[0089] [Figure 6] Shows the integration of an automatic switching extraction system into the propulsion equipment of an LNG carrier.

[0090] Preferred Embodiments of the Invention A preferred embodiment of a method for retrofitting a steam or hybrid propulsion installation of an LNG carrier is described below with the aid of FIGS.

[0091] 1 shows a modified LNG carrier steam or hybrid propulsion installation comprising a main condenser (5), an LNG carrier boiler (1), and a deaerator (53) connected to the main condenser (5) and the LNG carrier boiler (1). In addition, the LNG carrier boiler (1) comprises a combustion chamber (10), an exhaust gas outlet duct (18) connected to the combustion chamber (10), a high-pressure economizer (7) located inside the exhaust gas outlet duct (18), and a first steam superheater (4) located inside the combustion chamber (10).

[0092] The retrofit is carried out by connecting an auxiliary exhaust gas duct (23) to (18) after the high pressure economizer (7) located inside the existing gas outlet (18) and integrating a low pressure economizer (22) inside the auxiliary exhaust gas duct (23).

[0093] The low-pressure economizer (22) is configured to use the exhaust gas circulating inside the exhaust gas outlets (18, 23) to heat water going from the main condenser (5) to the deaerator (53), driven by an extraction pump (55) connected to the main condenser (5), when the steam or hybrid propulsion plant is operating at low loads, so that the amount of live steam required by the deaerator (53) to reach its design temperature is reduced accordingly.

[0094] FIG. 2 shows a modification of the tubes inside the LNG carrier boiler (1) by cutting sections of existing water tube walls (47) and replacing them with new curved tube (48) sections that overlap the remaining existing tubes (47) and creating an opening (50) in one wall of the combustion chamber (11) to allow the inflow of exhaust gas from a Gas Turbine Power Generator (GTPG) (30), thereby creating an opening (50) in the furnace (11).

[0095] Additionally, the LNG carrier boiler (1) uses the exhaust gas to maintain efficiency at low operating loads, and to produce steam and increase operating temperatures when the hybrid propulsion system operates at low loads, such as when operating at high operating loads.

[0096] Another advantage is that fuel consumption is reduced since no exhaust gases escape to the atmosphere, and the LNG carrier boiler (1) can operate as a HRSG (heat recovery steam generator) or a dual-fired boiler.

[0097] Complementing FIG. 2, FIG. 3 shows a cross-sectional view of the modification of the tubes inside the LNG carrier boiler (1) and a top view of the arrangement of the existing tubes (47) and the new curved tubes (48).

[0098] 4 shows the integration of a second steam superheater (35) inside the exhaust duct (8) of the GTPG (30) connected to the LNG carrier boiler (1) and leading into the opening (50). The second steam superheater (35) is integrated before the opening (50) formed in the LNG carrier boiler (1) to increase the temperature of the exhaust gas coming from the GTPG (30) so as to increase the operating temperature of the LNG carrier boiler (1) when the hybrid propulsion system is operating at low load.

[0099] On the other hand, FIG. 5 shows an integrated retrofit system (9) formed by the first water pipe (26), the second steam / water pipe (27), and the third steam pipe (28), as well as the steam / water separation drum (67).

[0100] FIG. 5 also shows components of a hybrid propulsion system, including a main condenser (5), an extraction pump (55) connected to the main condenser (5), a first stage water heater (65) connected to the extraction pump (55), a deaerator (53) connected to the first stage water heater (65), an LNG carrier boiler (1) connected to the deaerator (53), a high-pressure turbine (69) connected to the LNG carrier boiler (1), a low-pressure turbine (80) connected to the high-pressure turbine (69) by a crossover pipe (77), a low-pressure exhaust boiler (59) comprising a second low-pressure economizer (60), a saturated steam generator (61), and a low-pressure superheater (63), a DFDG (54) connected to the low-pressure exhaust boiler (59), and an auxiliary steam consumer (58).

[0101] The method includes integrating the retrofit system (9) and is based on connecting the main condenser (5) extracted water from a first stage water heater (65) to the second low-pressure economizer (60) by the first steam pipe (26) and the main condenser (5) extracted water from the second low-pressure economizer (60) to the deaerator (53) directly or through the steam / water separator drum (67) in order to heat the water coming from the main condenser (5) before it enters the deaerator (53).

[0102] The next step involves connecting the saturated steam generator (61) to the deaerator (53) by the second steam-water pipe (27) either directly or through the steam / water separation drum for heating the deaerator (53) water that feeds the LNG carrier boiler (1).

[0103] Finally, the deaerator (53) is connected to the low-pressure superheater (63) by the third steam pipe (28), and the low-pressure superheater (63) is connected to the crossover pipe (77) and the first steam consumer (58) in order to supply the steam derived from the low-pressure superheater (63) to the crossover pipe (77) and the first steam consumer (58), thus substituting the heating steam coming from the deaerator (53) or the steam / water separation drum (67) for the live steam supplied from the LNG carrier boiler (1).

[0104] The retrofit system (9) is integrated into selected piping points of the facility along with a new steam system, using heated water and steam produced in the low-pressure exhaust boiler (59) heated by the DFDG (54) exhaust gases to heat water coming from the main condenser (5) before entering the deaerator (53), using existing steam / air heaters to heat combustion air for the LNG carrier boiler (1), inject steam into a crossover (77) before the facility's low-pressure turbine (LPT) (80), and generally to replace auxiliary steam coming from the LNG carrier boiler (1).

[0105] Finally, FIG. 6 shows the integration of an automatic switching extraction system (40) into an existing extracted steam system, the automatic switching extraction system (40) comprising a first extracted steam line (32) connecting the first extracted steam system coming from the HPT (69) with a second extracted steam line (33) coming from the crossover (77), and a third extracted steam line (34) connecting the second extracted steam system coming from the crossover (77) with a third extracted steam system coming from the LPT (80).

[0106] The system also includes a plurality of automatic valves (70, 71, 72, 76, 78, 79, 81). The valves (70, 72) are opened after the existing first and second extraction automatic valves (71, 78) are closed. In this way, a set of second extraction steam consumers (74) are supplied with steam coming from the first extraction.

[0107] The other two automatic valves (76, 79) are opened after the automatic valves (78, 81) are closed, thus supplying the third extraction consumer (75) with steam coming from the second extraction.

[0108] The automatic changeover extraction system (40) is configured to control the steam extraction systems of the steam or hybrid propulsion facility during low load operation when the pressure in any steam extraction is too low to supply the corresponding system but the next lower pressure system can still be supplied. The automatic changeover extraction system (40) is connected to an automatic extraction control device (41) that controls the automatic changeover extraction system (40). [Brief explanation of the drawings]

[0109] [Figure 1] This shows a modified exhaust gas outlet of an existing LNG carrier boiler. [Figure 2] Modified tubes inside the combustion chamber of an LNG carrier boiler are shown. [Figure 3] 1 shows a cross section of the tubes inside the combustion chamber of an LNG carrier boiler. [Figure 4] The superheater is integrated in front of the LNG carrier boiler. [Figure 5] Shown is the integration of auxiliary steam and water systems within an LNG carrier propulsion installation. [Figure 6] The figure shows the integration of an automatic switching extraction system within the propulsion equipment of an LNG carrier.

Claims

[Claim 1] A method for modifying an auxiliary steam-water system of an LNG carrier equipped with steam or hybrid propulsion equipment, comprising: The propulsion equipment includes: A main condenser (5); an extraction pump (55) connected to the main condenser (5); a first stage water heater (65) connected to said extraction pump (55); a deaerator (53) connected to the first stage water heater (65); an LNG carrier boiler (1) connected to the deaerator (53); a high pressure turbine (69) connected to the LNG carrier boiler (1); a low-pressure turbine (80) connected to the high-pressure turbine (69) by a crossover pipe (77); a low-pressure exhaust boiler (59) including a second low-pressure economizer (60), a saturated steam generator (61), and a low-pressure superheater (63); a dual fuel diesel generator (DFDG) (54) connected to the low pressure exhaust gas boiler (59); a first auxiliary steam consumer (58); integrating a retrofit system (9) formed by a first water pipe (26), a second steam / water pipe (27), a third steam pipe (28), and a steam / water separation drum (67) into said propulsion installation; connecting, by the first water pipe (26), the main condenser (5) extraction water going from a first stage water heater (65) to the second low-pressure economizer (60) and the main condenser (5) extraction water going from the low-pressure economizer (60) to the deaerator (53) directly or through the steam / water separation drum (67) in order to heat the water coming from the main condenser (5) before entering the deaerator (53); connecting the saturated steam generator (61) to the deaerator (53) by the second steam / water pipe (27) directly or via the steam / water separation drum (67) for heating the water in the deaerator (53) that feeds the LNG carrier boiler (1); connecting the deaerator (53) to the low-pressure superheater (63) and connecting the low-pressure superheater (63) to the crossover pipe (77) and the auxiliary steam consumer (58) by the third steam pipe (28) in order to supply steam derived from the low-pressure superheater (63) to the crossover pipe (77) and the auxiliary steam consumer (58), thus substituting the heating steam coming from the deaerator (53) or the steam / water separation drum (67) for live steam supplied from the LNG carrier boiler (1).

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